Field of the Invention
The invention concerns a method for non-selective excitation of nuclear spin signals in an examination subject, as well as a magnetic resonance system, a non-transitory, computer-readable data storage medium encoded with programming instructions to implement such a method.
Description of the Prior Art
Magnetic resonance (MR) is a known modality with which images of the inside of an examination subject can be generated. Expressed simply, for this purpose the examination subject is positioned within a strong, static, homogeneous basic magnetic field (also called a B0 field) with field strengths of 0.2 Tesla to 7 Tesla and more, such that the nuclear spins of the examination subject orient along the basic magnetic field. To trigger magnetic resonance signals, radio-frequency excitation pulses (RF pulses) are radiated into the examination subject, the triggered nuclear magnetic resonance signals are measured, and MR images are reconstructed or spectroscopy data are determined based on these nuclear magnetic resonance signals. For spatial coding of the measurement data, rapidly switched magnetic gradient fields (called “gradients” for short) are superimposed on the basic magnetic field. The acquired measurement data are digitized and stored as complex numerical values in a k-space matrix. An associated MR image can be reconstructed from the k-space matrix populated with such values, for example by means of a multidimensional Fourier transformation.
In the triggering of the magnetic resonance signals, the spins located in the examination region are excited out of their rest state and (from a classical consideration) tipped or flipped into the transverse plane. This transverse magnetization can be measured by induction.
In the excitation, a distinction can be made between selective RF excitation pulses (that, for example, only excite one slice in the examination subject) and non-selective RF excitation pulses. Non-selective RF excitation pulses optimally uniformly excite the entire examination subject, or at least the examination region to be examined in the examination subject. Additional gradients are switched for spatial resolution. For example, for a resolution in the slice direction, gradients are switched in the slice direction.
MR sequences are known that can use non-selective RF excitation pulses. For example, the known RASP sequence as described by Heid and Deimling in “Rapid Signal Point (RASP) Imaging”, SMR, 3rd Annual Meeting, Page 684, 1995. Additional examples are, for example, turbo spin echo sequences or even what is known as the UTE (“Ultrashort Echo Time”) sequence, for example as described in the article by Sonia Nielles-Vallespin, “3D radial projection technique with ultrashort echo times for sodium MRI: Clinical applications in human brain and skeletal muscle”, Magn. Res. Med. 2007; 57; P. 74-81.
In some of these sequences, different gradients are already switched at the point in time of the excitation in order to make the measurement as quiet as possible, i.e. to minimize noise development due to the gradient switchings. In order to ensure a uniform excitation of the examination subject, the spectral bandwidth of the RF excitation pulse that is used must be relatively high, and the duration of the RF excitation pulse must be as short as possible.
If the spectral width of the non-selective RF excitation pulse is not sufficient in order to excite all spins in the examination subject at every gradient switching, in defined gradient configurations the outer regions of the examination region to be imaged are not excited, or are excited only to a limited extent. This leads to a blurring of the affected regions of the examination region in the MR images reconstructed from the measurement data. For measurements of examination regions whose center does not coincide with the isocenter of the magnetic resonance system (thus with the center of the measurement volume of the magnetic resonance system), such blurring artifacts increase due to the absence of excitation via the RF excitation pulse. Such measurements at examination regions to the side of the isocenter—for example measurements of the hand or the elbow (among other things) of a patient—are also designated as “off-center measurements”.
This problem has previously been ignored, for the most part. At best, the strength of the gradients is reduced. However, imaging-relevant variables such as the readout bandwidth, the repetition time TR and the contrast of the image change with this. An expansion of the spectral excitation width of an RF excitation pulse by reducing the duration of the RF excitation pulse simultaneously reduces the maximum achievable excitation flip angle and the precision of the actually emitted RF excitation pulse proportional to the duration of the RF excitation pulse, and therefore likewise is limited in its application.